This invention relates to processes and apparatuses for the fluidized contacting of catalyst with hydrocarbons. More specifically, this invention relates to processes and apparatuses for stripping entrained or adsorbed hydrocarbons from catalyst particles.
A variety of processes contact finely divided particulate material with a hydrocarbon containing feed under conditions wherein a fluid maintains the particles in a fluidized condition to effect transport of the solid particles to different stages of the process. Catalyst cracking is a prime example of such a process that contacts hydrocarbons in a reaction zone with a catalyst composed of finely divided particulate material. The hydrocarbon feed fluidizes the catalyst and typically transports it in a riser as the catalyst promotes the cracking reaction. As the cracking reaction proceeds, substantial amounts of hydrocarbon, called coke, are deposited on the catalyst. A high temperature regeneration within a regeneration zone burns coke from the catalyst by contact with an oxygen-containing stream that again serves as a fluidization medium. Coke-containing catalyst, referred to herein as spent catalyst, is continually removed from the reaction zone and replaced by essentially coke-free catalyst from the regeneration zone. Fluidization of the catalyst particles by various gaseous streams allows the transport of catalyst between the reaction zone and regeneration zone. Methods for cracking hydrocarbons in a fluidized stream of catalyst, transporting catalyst between reaction and regeneration zones and combusting coke in the regenerator are well known to those skilled in the art of FCC processes. To this end, the art is replete with vessel configurations for contacting catalyst particles with feed and regeneration gas, respectively.
A majority of the hydrocarbon vapors that contact the catalyst in the reaction zone are separated from the solid particles by ballistic and/or centrifugal separation methods within the reaction zone. However, the catalyst particles employed in an FCC process have a large surface area, which is due to a great multitude of pores located in the particles. As a result, the catalytic materials retain hydrocarbons within their pores, upon the external surface of the catalyst and in the spaces between individual catalyst particles as they enter the stripping zone. Although the quantity of hydrocarbons retained on each individual catalyst particle is very small, the large amount of catalyst and the high catalyst circulation rate which is typically used in a modern FCC process results in a significant quantity of hydrocarbons being withdrawn from the reaction zone with the catalyst.
Therefore, it is common practice to remove, or strip, hydrocarbons from spent catalyst prior to passing it into the regeneration zone. Greater concentrations of hydrocarbons on the spent catalyst that enters the regenerator increase its carbon-burning load and result in hotter regenerator temperatures. Avoiding the unnecessary burning of hydrocarbons is especially important during the processing of heavy (relatively high molecular weight) feedstocks, since processing these feedstocks increases the deposition of coke on the catalyst during the reaction (in comparison to the coking rate with light feedstocks) and raises the temperature in the regeneration zone. Improved stripping permits cooler regenerator temperatures. Stripping hydrocarbons from the catalyst also allows recovery of the hydrocarbons as products.
The most common method of stripping the catalyst passes a stripping gas, usually steam, through a flowing stream of catalyst, counter-current to its direction of flow. Such steam stripping operations, with varying degrees of efficiency, remove the hydrocarbon vapors which are entrained with the catalyst and adsorbed on the catalyst. Contract of the catalyst with a stripping medium may be accomplished in a simple open vessel as demonstrated by U.S. Pat. No. 4,481,103 B1.
The efficiency of catalyst stripping is increased by using vertically spaced baffles to cascade the catalyst from side to side as it moves down a stripping apparatus and counter-currently contacts a stripping medium. Moving the catalyst horizontally increases contact between the catalyst and the stripping medium so that more hydrocarbons are removed from the catalyst. In these arrangements, the catalyst is given a labyrinthine path through a series of baffles located at different levels. Catalyst and gas contact is increased by this arrangement that leaves no open vertical path of significant cross-section through the stripping apparatus. Further examples of these stripping devices for FCC units are shown in U.S. Pat. No. 2,440,620 B1, U.S. 2,612,438 B1, U.S. 3,894,932 B1, U.S. 4,414,100 B1 and U.S. 4,364,905 B1. These references show the typical stripping vessel arrangement having a stripping vessel, a series of outer baffles in the form of frusto-conical sections that direct the catalyst inwardly onto a series of inner baffles. The inner baffles are centrally located conical or frusto-conical sections that divert the catalyst outwardly onto the outer baffles. The stripping medium enters from below the lower baffles and continues rising upwardly from the bottom of one baffle to the bottom of the next succeeding baffle. Variations in the baffles include the addition of skirts about the trailing edge of the baffle as depicted in U.S. Pat. No. 2,994,659 B1 and the use of multiple linear baffle sections at different baffle levels as demonstrated in FIG. 3 of U.S. Pat. No. 4,500,423 B1. A variation in introducing the stripping medium is shown in U.S. Pat. No. 2,541,801 B1 where a quantity of fluidizing gas is admitted at a number of discrete locations.
Currently in stripping vessels for FCC units, the baffles are typically oriented to have an angle of 45xc2x0 with respect to the horizontal. The sloped baffles assure that catalyst moves off the tray down to the next level in the stripping vessel. However, because the sloped trays each occupy substantial elevation, they limit the number of trays that can be installed in a given height of a stripping vessel. The greater the number of trays in the stripping vessel, the greater the overall performance. Moreover, sloped baffles generate a differential pressure head between holes that are lower in elevation on a baffle compared to the holes which are higher in elevation on the baffle. Because the pressure is going to be greater at lower elevations on the baffle, the velocity through the jets on the baffle will be greater at higher elevations on the baffle. This makes hydraulics through the stripping vessel more difficult to control. Moreover, erosion occurs through the jets which are higher on the baffle than through jets that are lower on the baffle because of the velocity differential. Consequently, the variously eroded holes exacerbate the difficulty in controlling hydraulics. On the other hand, setting baffles at a smaller slope will result in catalyst accumulation on top of the baffle unless fluidization over the baffle is increased, which could require increasing the flow rate of stripping medium.
It is an objective of any new stripping design to minimize the addition of stripping medium while maintaining the benefits of good catalyst stripping throughout the FCC process unit. In order to achieve good stripping of the catalyst with the resultant increased product yield and enhanced regenerator operation, relatively large amounts of stripping medium have been required. For the most common stripping medium, steam, the average requirement throughout the industry is about 2 kg of steam per 1000 kg (2 lbs. of steam per 1000 lbs.) of catalyst for catalyst stripping. In the case of steam, the costs include capital expenses and utility expenses associated with supplying the steam and removing the resulting water via downstream separation facilities. Where there is not adequate supply or treatment capacity, the costs associated with raising the addition of stripping medium can be significant. In such cases, achieving better stripping without an increase in the required steam will yield substantial economic benefits to the FCC process.
However, better stripping brings more important economic benefits to the FCC process by reducing xe2x80x9cdelta cokexe2x80x9d. Delta coke is the weight percent coke on spent catalyst less the weight percent coke on regenerated catalyst. Reducing delta coke in the FCC process permits a lowering of the regenerator temperature. More of the resulting, relatively cooler regenerated catalyst is required to supply the fixed heat load in the reaction zone. Hence, the reaction zone may operate at a higher catalyst-to-feed or catalyst-to-oil (C/O) ratio. The higher C/O ratio increases conversion which increases the production of valuable products. Consequently, improved stripping results in improved conversion. A stripping operation that reduces the production of delta coke by 0.05 wt-% can lower the regenerator temperature by xe2x88x929xc2x0 to xe2x88x927xc2x0 C. (15xc2x0 to 20xc2x0 F.) and permit a C/O ratio increase in the range of 6%. The corresponding improvement in conversion yields 0.6 to 0.7 vol-% more gasoline as well also increasing the yield of desired light products. Therefore, it is a further objective of this invention to decrease delta coke by more efficient catalyst stripping.
Moreover, it is not possible to simply increase stripping efficiency or capacity by accepting the economic penalties associated with the use of increasing amounts of steam. At some point, the typical stripping vessel that operates with baffles becomes limited by the amount of catalyst flux moving through the stripping vessel. A practical limit on catalyst flux for operating such stripping vessels is approximately 439,380 kg/hr/m2 (90,000 lbs/hr/ft2) based on total area of the stripping vessel. Attempts have been made to increase the capacity and effectiveness of stripping in a baffle-style stripping unit by modifying the configuration and area of the baffles. U.S. Pat. No. 5,531,884 B1 shows a modification to a baffle-style stripping vessel that incorporates one or more rings of large vertical conduits to provide an additional catalyst and gas circulation path across the baffles. It is also known to concentrate openings in a very centralized portion of the stripping baffles.
It has now been found that providing a stripping vessel having horizontal baffles with downcomers will provide improved stripping efficiency and catalyst flux through the stripping vessel. It was also unexpectedly found that the stripping efficiency increases with higher catalyst flux when using the horizontal baffles with downcomers of this invention. The utilization of downcomers generates sufficient transverse movement of the catalyst across each baffle to allow the stripping medium to rise through the catalyst and provide better mixing. Completely distributing relatively small openings over the entire surface of a section of the baffle has been found to sufficiently fluidize catalyst on the baffle at high catalyst flux rates. Moreover, more horizontal baffles can be installed in a stripping vessel of a given height, thereby improving stripping performance. By this discovery, previous limits for typical baffle-type stripping vessel throughput may be increased by at least 22% by installing inexpensive stripping baffles. The process of this invention has benefits at all flux rates, has particular benefits at flux rates of at least 292,920 kg/hr/m2 (60,000 lbs/hr/ft2) of stripping vessel area and is particularly useful at flux rates of at least 537,020 kg/hr/m2 (110,000 lbs/hr/ft2) of stripping vessel area. Moreover, the improved stripping design achieves high stripping efficiencies at very low steam rates, which reduces overall FCC unit operating costs.
In one embodiment, the present invention relates to a process for the stripping of entrained and/or adsorbed hydrocarbons from particulate material in a stripping vessel. The process comprises contacting particles with a hydrocarbon stream. Hydrocarbons are disengaged from the particles after contact with the hydrocarbon stream to produce a stream of contacted particles containing entrained or adsorbed hydrocarbons. The contacted particles are passed downwardly through a plurality of stripping baffles. Each baffle has a slope of less than or equal to 10xc2x0 with respect to the horizontal and extends across less than the entire cross sectional area of the stripping vessel to define a downcomer section. A stripping fluid is discharged upwardly through a plurality of openings distributed over each stripping baffle to strip hydrocarbons from the particulate material. Stripping fluid, stripped hydrocarbons and stripped particles are recovered from the stripping baffles.
In another embodiment, the present invention relates to an apparatus for the stripping of entrained and/or adsorbed hydrocarbons from particulate material. The apparatus comprises a stripping vessel. The stripping vessel defines at least one port for receiving particles that contain entrained or adsorbed hydrocarbons from the contact of the particles with a hydrocarbon stream and for withdrawing stripping fluid and stripped hydrocarbons from the stripping vessel. A plurality of stripping baffles are spaced apart vertically over at least a portion of the stripping vessel height with each baffle having a slope of no more than 10xc2x0. Each of the stripping baffles extends over less than the entire horizontal, cross-sectional area of the stripping vessel to define a downcomer section. A plurality of openings are distributed over the surface of each stripping baffle. The apparatus comprises at least one fluid inlet for passing a stripping fluid to the underside of at least one stripping baffle for stripping hydrocarbons from the particulate material and at least one particle outlet for recovering stripped particles from the stripping baffles.
In a further embodiment, the present invention relates to an apparatus for the stripping of entrained and/or adsorbed hydrocarbons from particulate material. The apparatus comprises a stripping vessel. The stripping vessel defines at least one port for receiving particles that contain entrained or adsorbed hydrocarbons from the contact of the particles with a hydrocarbon stream and for withdrawing stripping fluid and stripped hydrocarbons from the stripping vessel. A plurality of stripping baffles are spaced apart vertically over at least a portion of the stripping vessel height with each baffle having a slope of no more than 10xc2x0. Each of the stripping baffles extend over less than the entire horizontal, cross-sectional area of the stripping vessel to define a downcomer section. A plurality of openings are distributed over the surface of each stripping baffle. The apparatus comprises at least one fluid inlet for passing a stripping fluid to the underside of at least one stripping baffle for stripping hydrocarbons from the particulate material and at least one particle outlet for recovering stripped particles from the stripping baffles.
Accordingly, it is an object of this invention to increase the maximum capacity at which a baffle-style stripping vessel may operate.
It is another object of this invention to increase the efficiency of stripping in a baffle style stripping vessel.
It is a further object of this invention to obtain a method and apparatus that provides a more complete and reduced utilization of stripping medium.
Additional objects, embodiments and details of this invention are given in the following detailed description of the invention.